Thin client server has attracted public attention in view of enhanced security and desktop portability.
One possible form of virtual machine (VM)-based thin client server (for example, VirtualPCCenter), typically adopted by enterprise data center, allows paid software represented by Microsoft Office (registered trademark) to run thereon.
This sort of environment suffers from a risk that VM images are replicated by a malicious user or system administrator, and leaked towards a large unspecified number of people. This is because it is easy to illegally copy or alter digital contents, and also because it is easy to readily distribute digital contents over a wide range through a network, by virtue of dissemination of file conversion (P2P, or Peer-to-Peer) technique.
Accordingly, once the digital contents such as VM image including paid software are illegally leaked, it is necessary to trace them.
In recent years, techniques for tracing such digital contents have been developed (see Non-Patent Document 1, Non-Patent Document 2, for example).
Embedding of a digital watermark into the digital contents may be exemplified as one technique of tracing the digital contents. The digital watermark is a technique of embedding identification information into the digital contents, without degrading quality thereof.
Even for the case where the digital contents were illegally leaked by breaking tamper resistant technique or encryption technique for blocking license violation, the leaked digital contents can be traced by using the digital watermark.
In recent years, a technique for tracing such digital contents has been developed (see Non-Patent Document 1, for example). For digital contents relevant to pictures and sounds, a large number of digital watermarking techniques have been developed.
According to Non-Patent Document 1, those making use of frequency range or picture replacement are exemplified for picture contents, and those making use of echo or phase modification are exemplified for sound contents.